Colloidally synthesized nanocrystals, e.g. semiconductor nanocrystals, such as quantum dots (QD) are a class of materials that offer various promising applications in fields related to light emission and absorption, e.g. in fields such as in-vivo imaging, light-emitting device manufacture, photodetection and solar energy conversion. For example, quantum dots may find application in transistors, solar cells, LEDs, diode lasers, medical imaging, quantum computing and a variety of other fields. Furthermore, QDs emitting in the visible electromagnetic spectrum may be of particular interest for lighting and display applications, e.g. for high brightness LEDs. A quantum dot may be sufficiently small to exhibit distinct quantum mechanical properties. A single QD can for example contain about 100 to even 100000 atoms, having a diameter that ranges from about 10 to 50 or more atoms, e.g. a diameter in the range of about 2 to about 10 nanometers. For example, three-dimensional confinement of the nanocrystal exciton states can be achieved, such that intermediate properties are obtained between those of the bulk material and discrete molecules. Therefore, the characteristics of a quantum dot may be closely related to its size and shape, e.g. the band gap, which determines the frequency range of emitted light, may be inversely related to its size.
Monodisperse ensembles of QDs may feature a narrow, size-tunable emission spectrum in combination with a broad absorption and excitation spectrum, while also being particularly suitable for solution-based processing. Colloidal synthesis of nanocrystals may comprise the synthesis from precursor compounds in a solution. When heating the solution, the decomposed precursors form monomers that nucleate. Known QDs obtainable by colloidal synthesis may comprise binary compounds, such as lead sulfide, lead selenide, cadmium selenide, cadmium sulfide, indium arsenide and indium phosphide, or ternary compounds such as cadmium selenide sulfide. Particular nanocrystals known in the art may involve cadmium chalcogenide based materials where especially CdSe QDs synthesis is a fully mastered process. Such nanocrystal can be easily manufactured due to the simplicity of their synthesis, and may have a high optical quality. However, cadmium is a toxic heavy element which may be subject to legal restrictions, e.g. by the EU ROHS directive. Consequently, the use of such materials in large amounts is preferably avoided. Furthermore, the high toxicity may limit the applicability of cadmium chalcogenide based materials, e.g. in in-vivo imaging. In addition to a low toxicity, a cost-efficient production at an industrial scale can also be considered highly advantageous when scaling the use of nanocrystals from an academic setting to commercial product applications.
Cadmium-free alternatives for manufacturing quantum dots are known in the art, such as CuInS2 and InP. Particularly indium phosphide (InP) QDs are known that have emission characteristics similar to CdSe QDs, while advantageously having a lower toxicity. Methods for colloidally manufacturing InP nanocrystals are known in the art. For example, a first group of known synthesis methods may use a highly reactive phosphorous precursor, e.g. P(−III) as tris(trimethylsylil)phosphine (TMS)3P or phosphine PH3. A second group of known synthesis methods may use a phosphorous precursor with a lower reactivity, e.g. P(0) or P(+III) as trioctylphosphine TOP, P4 or PCl3. Highly reactive precursors may provide a better size dispersion, which may be an important parameter to obtain QD dispersions suitable for optical devices.
Particularly (TMS)3P may be commonly used as phosphorous precursor, as it may offer good quality in terms of the properties of the resulting InP nanocrystals. However, this compound may have some disadvantages, e.g. a relatively high cost, pyrophoricity and the production of PH3 in contact with air, which is a highly toxic gas. These disadvantages may also hamper the production of InP nanocrystals at an industrial scale. PH3-based synthesis may also give good results in terms of size dispersion, but has the disadvantage of the high toxicity of PH3. Therefore, PH3 may also be difficult and expensive to use for QD production at a larger scale.
While phosphorous precursors with a low reactivity may provide a less than optimal size dispersion in accordance with synthesis methods known in the art, e.g. may produce nanocrystals with a large size-dispersion which may be difficult to use for various potential applications, these precursors have the advantage of being cheap and easy to use. Synthesis methods known in the art using such low reactivity phosphorous precursors may rely on a two-step method, e.g. in which at least the indium precursor is reduced before reacting with the phosphorous precursor. For example, InCl3 can be reduced by KBH4 to form In0 that reacts with P0, or InCl3 can be reduced by LiR before reacting the In0 with TOP by a catalytic cleavage at high temperature.
The Unites States patent application US 2014/084212 discloses a method for synthesizing InP nanoparticles using an indium trichloride anhydride as an indium raw material and tris(dimethylamino)phosphine (DMA)3P as a phosphorus raw material. This method includes a preparation step of mixing the indium raw material, the phosphorus raw material, an organic solvent having a boiling point of 170° C. or higher, and an aliphatic amine having a carbon number of 18 or more as a particle surface ligand to obtain a mixture solution, and a synthesis step of synthesizing the InP nanoparticles by heating the mixture solution to 150° C. or higher, but lower than 170° C. (DMA)3P is a low-reactive phosphorous precursor (P(+III)) which is stable under air, and may cost considerably less than (TMS)3P.
Song et al. also have disclosed a related method for the manufacture of InP QDs in Nanoparticle Res. 15, pp. 1750. With this protocol, InP/ZnS core/shell nanoparticles may be produced with an acceptably low size dispersion. For colloidal semiconductor QDs, the size dispersion is in general directly reflected in the width of the emission spectrum. Song et al. reported InP QDs with a full width at half maximum (FWHM) of 60-65 nm, which may be considered close to the 40-60 nm range of FWHM obtainable with (TMS)3P. Therefore, this method of InP QDs synthesis may combine the advantages of low-reactive phosphorous precursors with good size dispersion.
It is also known in the art to tune the size of the colloidal nanocrystals that are produced, such as to obtain the desired optical and/or electronic properties. A known method to obtain different sizes is to stop the nanocrystals growth during the synthesis. However, a disadvantage of this approach is that the chemical yield for small nanocrystals sizes can be very low because the reaction is not complete. However, methods for high yield size-tuning are also known in the art for synthesis of CdSe or CdS nanocrystals by varying the concentration of the precursors and/or the solute solubility accordingly.